In the preparation of typical composite solid propellants, a liquid crosslinkable polymeric substance is used as a vehicle for the dispersing of the particulate oxidizer material, burning rate catalyst, solid fuel, plasticizer and other materials normally incorporated into a crosslinked composite propellant. A crosslinking agent for the liquid polymer is added to the system and, after mixing, the mixed uncured propellant is cast into a suitable form and cured under the desired conditions of temperature and pressure. State-of-the-art propellant systems normally employ functionally active polymers such as the polybutadiene-acrylic acid-acrylonitrile polymers (PBAN), epoxy, hydroxy, and carboxy functional polybutadienes, polyisobutylenes, polyethers and polyesters. The crosslinking agents for these materials are commercially available compounds having the activity required by the particular functional group chosen. For example, if carboxyfunctionality is used in the polymer, an aziridinyl or epoxy crosslinking agent could be used. If hydroxyfunctionality is used in the polymer, an isocyanate crosslinking agent could be used. If epoxyfunctionality is used in the polymer, a carboxy crosslinking agent could be used all as is known to the art.
In the manufacture of these propellants, it is sometimes necessary to add combustion catalysts which derive their activity from the presence of iron. The iron may be present in the form of iron oxide or as ferrocene or a ferrocene derivative, for example. Other combustion catalysts involve the use of copper oxide, copper salts and compounds of various transition metals. Further, due to the equipment and ingredients used in the processing of a propellant, it is almost impossible to obtain a mix which is totally free of these metals even if compounds of these materials are not specifically added to the mix for any particular purpose.
It has been found that the presence of these metals can so accelerate the curing action of the propellant that it is impossible to mix the material together to the desired degree of dispersion without the viscosity increasing to the point where it is no longer possible to cast the mixture into a suitable configuration. It has also been found that even if the pot life can be extended sufficiently to permit the casting of the propellant, the presence of these metals tends to promote the subsequent degradation of the cured binder. This degradation is most commonly manifested by discoloration, hardening, or embrittlement of the propellant, particularly when the propellant is stored at temperatures higher than ambient in the presence of atmospheric oxygen. It appears most likely that these metals cause the undesirable effects on the cure by a catalysis of the cure reaction and the degradation of the propellants by the catalysis of oxidation reactions.
According to this invention, the undesirable effects of these metals may be alleviated without destroying their effectiveness as combustion catalysts, if they have been added for that purpose, by adding a binder soluble agent to the binder mixture which agent is capable of reacting with the undesirable metals to form stable complexes in which the valence of the metal ion is fixed. This renders the metal inactive as a cure or oxidation catalyst without affecting the activity as a combustion catalyst.